The importance of muscle training is well known. Muscle training can be used to increase the strength, general fitness, or rehabilitate after an injury. Gymnasiums may have exercise machines using weights and pulleys but the size and expense of such devices can lead to non-use. Free weights are cumbersome, are difficult to store, and require many different weights to be effective. Known devices that use springs or elastic bands provide resistance to muscle contraction but can limit the muscles affected, or lack the ability to isolate muscles.
What is lacking in the art is a lightweight, portable device capable of use in muscle training for particular muscles.